Galvanized steel sheets are widely applied for various uses because of the excellent corrosion resistance imparted to a steel sheet under the effect of sacrificial protection (also referred to as cathodic protection) of the galvanized layer. An alloy-treated galvanized steel sheet, of which the entire galvanized layer is converted into a zinc-iron alloy layer (hereinafter referred to as a "Zn-Fe alloy layer") through a heating treatment after the galvanizing treatment is now attracting again the general attention in terms of the excellent corrosion resistance after coating (hereinafter referred to as the "post-coating corrosion resistance"), and uses thereof are expanding to cover those in automobiles and home electrical appliances. Especially, in addition to the high post-coating corrosion resistance, the excellent weldability and chipping resistance are creating an increasing demand for the alloy-treated galvanized steel sheet for use in external, underside and closed structures of automobiles for preventing damage caused by salt in automobiles used in cold districts where ice and snow on the road are melted with salt in winter.
There are known the following processes for manufacturing an alloy-treated galvanized steel sheet mentioned above:
(1) Processes based on hot-dip galvanizing:
(a) A process for manufacturing a hot-dip galvanized steel sheet having a Zn-Fe alloy galvanized layer, disclosed in Japanese Patent Publication No. 3,107/62 dated May 31, 1962, which comprises:
passing a steel sheet through a molten zinc bath in a non-oxidizing atmosphere; withdrawing said steel sheet from said bath while removing excess zinc from the surface of said steel sheet; and, heating said steel sheet, on the surface of which a galvanized layer has been formed, to alloy said galvanized layer and said steel sheet; thereby converting said galvanized layer into a Zn-Fe alloy layer;
said process being characterized by:
electrically heating said steel sheet from inside thereof to a temperature above the melting point of pure zinc, while said galvanized layer is still in the molten state; and, converting said galvanized layer into a uniform Zn-Fe (8-12%) alloy layer by holding said heating temperature during a period of time sufficient to allow iron molecules to move into said galvanized layer;
said process including also a step of adding aluminum in a slight amount into said molten zinc bath (hereinafter referred to as the "prior art (1)").
(b) A process for manufacturing an alloy-treated hot-dip galvanized steel sheet, disclosed in Japanese Patent provisional Publication No. 21,940/75 dated Mar. 8, 1975, which comprises:
Passing a steel sheet through a molten zinc bath added with at least 0.05 wt.% aluminum to prepare a hot-dip galvanized steel sheet; and then, converting the galvanized layer of said galvanized steel sheet into a Zn-Fe (2-60%) alloy layer by heating said galvanized steel sheet at a temperature of from 400.degree. to 480.degree. C. in a batch annealing furnace (hereinafter referred to as the "prior art (2)").
(2) Process based on electro-galvanizing:
(a) A process for manufacturing an electro-galvanized steel sheet excellent in corrosion resistance and paintability, disclosed in Japanese Patent Provisional Publication No. 42,343/72 dated Dec. 15, 1972, which comprises:
subjecting a cold-rolled steel sheet to a conventional electro-galvanizing treatment to prepare an electro-galvanized steel sheet having a galvanized layer with a thickness of from 0.5 to 10 .mu.m per side; then, converting said galvanized layer into a Zn-Fe (2-9%) alloy layer by heating said electro-galvanized steel sheet at a temperature of from 350.degree. to 800.degree. C. for a period of time of from 1 to 30 seconds; and then, rapidly cooling said electro-galvanized steel sheet (hereinafter referred to as the "prior art (3)").
(b) A process for manufacturing an electro-galvanized steel sheet for coating, disclosed in Japanese Patent Provisional Publication No. 154,126/75 dated Dec. 11, 1975, which comprises:
subjecting a steel sheet to a conventional electro-galvanizing treatment to prepare an electro-galvanized steel sheet having a galvanized layer with a thickness of from 1 to 3 .mu.m per side; then, converting said galvanized layer into a Zn-Fe (6-9%) alloy layer by heating said electro-galvanized steel sheet to a temperature of from 450.degree. to 600.degree. C. at a heating rate of from 2.degree. to 60.degree. C./second in a reducing or neutral atmosphere; and then, rapidly cooling said electro-galvanized steel sheet (hereinafter referred to as the "prior art (4)").
(c) A process for manufacturing a one-side electro-galvanized steel sheet, disclosed in Japanese Patent Provisional Publication No. 17,534/78 dated Feb. 17, 1978, which comprises:
subjecting a cold-rolled steel sheet to a conventional one-side electro-galvanizing treatment to prepare a one-side electro-galvanized steel sheet having a galvanized layer in an amount of from 10 to 40 g/m.sup.2 per side; then, after coiling said electro-galvanized steel sheet into an open coil, converting said galvanized layer into a Zn-Fe (6.5-25%) alloy layer by heating said electro-galvanized steel sheet in the state of an open coil at a temperature of from 250.degree. to 375.degree. C. for a period of time of from 0.1 to 20 hours in a non-oxidizing or weak reducing atmosphere; and then, subjecting said electro-galvanized steel sheet to a skinpass rolling at a reduction ratio of about 1% (hereinafter referred to as the "prior art (5)").
Conventional alloy-treated galvanized steel sheets manufactured by the above-mentioned prior arts (1) to (5), of which the entire galvanized layer is converted into a Zn-Fe alloy layer in all cases, have certainly a post-coating corrosion resistance superior to that of a cold-rolled steel sheet and an ordinary galvanized steel sheet not applied with an alloying treatment, but have the following problems:
(1) In a conventional alloy-treated galvanized steel sheet, which contains Fe in the galvanized layer thereof, bare corrosion resistance of the galvanized layer, i.e., corrosion resistance of the galvanized layer itself is inferior to that of an ordinary galvanized steel sheet not applied with an alloying treatment. More particularly, corrosion of a coated galvanized steel sheet starts from a flaw of the coated film, a portion without coated film because of the insufficient adhesion of the coated film and a portion where a film of a required thickness has not been ensured. An inferior bare corrosion resistance of the galvanized layer causes rapid progress of corrosion from the above-mentioned portions with defective coating, thus seriously impairing corrosion resistance of the galvanized steel sheet as a whole.
(2) A galvanized steel sheet is usually subjected to various formings such as bending and drawing to meet the final use. To have an excellent formability is therefore one of the important properties indispensable for a galvanized steel sheet. However, because the entire galvanized layer is converted into a hard and brittle Zn-Fe alloy layer in the conventional alloy-treated galvanized steel sheet, formability is seriously decreased according as the galvanized layer grows thicker, thus making it impossible for the conventional alloy-treated galvanized steel sheet to bear a severe forming. On the other hand, if the galvanized layer is made thinner to prevent formability from decreasing, corrosion resistance is decreased. For these reasons, the thickness of the galvanized layer of the conventional alloy-treated galvanized steel sheet is inevitably limited within a certain range.
(3) In the hot-dip galvanizing process, as in the prior arts (1) and (2), not only it is difficult to apply a thin galvanized layer and a one-side galvanized layer, but also the thickness distribution of the galvanized layer tends to be non-uniform. Furthermore, in the conventional alloy-treated galvanized steel sheet manufactured by the hot-dip galvanizing process, the high temperature of the molten zinc bath (about 460.degree. C.) causes deterioration of the properties such as ductility and deep-drawability of the steel sheet, thus restricting the uses within a limited range.
(4) The electro-galvanizing process, as in the prior arts (3) to (5), requires a high installation cost because of the necessity to specially install a heating equipment with an adjusted atmosphere in or outside the line. As in the prior art (5), furthermore, a heating step applied after coiling the electro-galvanized steel sheet into an open coil makes the process more complicated, thus leading to a higher manufacturing cost.
As compared with the conventional alloy-treated galvanized steel sheet manufactured in compliance with any of the prior arts (1) to (5), the ordinary galvanized steel sheet not applied with an alloying treatment, having a higher formability, is inferior in post-coating corrosion resistance and is not therefore suitable for external, underside and closed structures of an automobile.
Post-coating corrosion resistance of a galvanized steel sheet is generally considered to be determined by respective corrosion resistance of the galvanized layer, the chemical film or the painted film and corrosion resistance of the interface between the galvanized layer and the chemical film or the painted film. Particularly, in a galvanized steel sheet for coating, bare corrosion resistance of the galvanized layer and corrosion resistance of the interface between the galvanized layer and the chemical film or the painted film are very important for ensuring a satisfactory post-coating corrosion resistance.
However, the conventional galvanized steel sheet applied with an alloying treatment for the purpose of improving post-coating corrosion resistance, while being excellent in corrosion resistance of the interface between the galvanized layer and the chemical film or the painted film, is inferior in bare corrosion resistance of the galvanized layer and formability. The ordinary galvanized steel sheet, on the contrary, while being excellent in bare corrosion resistance of the galvanized layer and formability, is very low in corrosion resistance of the interface between the galvanized layer and the chemical film or the painted film.
Under such circumstances, there is an demand for the development of an electro-galvanized steel sheet for coating which is excellent not only both in bare corrosion resistance of the galvanized layer and corrosion resistance of the interface between the galvanized layer and the chemical film or the painted film, but also in formability, but an electro-galvanized steel sheet provided with all these properties is not as yet proposed.